The present invention relates to manufacturing semiconductor substrates, more particularly to a system and method for manufacturing crystals using a magnetic field furnace.
Dendritic web ribbon crystals are commonly used as substrates for solar cells because of their high chemical purity, low density of structural defects, rectangular shape, and relatively thin crystal size. Furthermore, solar cells fabricated from dendritic web silicon possess light energy to electrical energy conversion efficiencies as high as 17.3%, which is comparable to high efficiencies obtained using expensive processes such as Float Zone silicon and other well-known complex processes.
FIG. 1 illustrates a ribbon or sheet of a dendritic web silicon crystal 10. Dendritic web silicon crystal 10 is withdrawn as a single crystal from a first silicon melt region 12A. Second silicon melt regions 12B are separated from first melt region 12A by barriers 14. Barriers 14 are implemented to provide some measure of thermal isolation between first and second silicon melt regions 12A and 12B. Small openings (not illustrated) in barriers 14 allow molten silicon to flow from second melt regions 12B to first melt region 12A. By maintaining first melt region 12A just below silicon""s melting point, crystal continually freezes in first melt region 12A. Second melt regions 12B become replenished by heating it just above the melting point and mechanically feeding silicon pellets into second melt regions 12B. First and second silicon melt regions 12A and 12B are contained in a crucible 16.
Silicon crystal 10 is typically grown by pulling a seed 18 at an upwardly direction at a speed of approximately 1.8 cm/min. The resulting dendritic web silicon crystal 10 includes a silicon web portion 20 bounded by silicon dendrites 22. Web portion 20 is typically about 3 to 8 cm in width and about 100 xcexcm in thickness compared to the nominally square dendrites, which are typically about 550 xcexcm thick. In order to sustain the above described crystal growth, the dendrite support structure is continually regenerated at pointed dendrite tips 24 beneath the surface of the melt contained in first melt region 12A.
The conventional dendritic web crystal growth processes suffer from several drawbacks such as xe2x80x9cmetastablility,xe2x80x9d which causes premature termination of crystal growth. Crystal lengths of only one or two meter can be achievedxe2x80x94which are commercially impractical to produce. To provide a commercially improved product, it was discovered that the application of a magnetic field to the melt, from which the crystal is drawn, produces improvements, including stabilization of dendritic web crystal growth. A patent application entitled xe2x80x9cMethod and System for Stabilizing Dendritic Web Crystal Growth,xe2x80x9d Ser. No. 09/294,529, filed on Apr. 19, 1999, assigned to the assignee of the present invention, and incorporated herein by reference, describes the application of a magnetic field to a dendritic web crystal growth. One example of such magnetic field is illustrated in FIG. 2. FIG. 2 illustrates a furnace chamber 30 having a dipole magnet which includes a pair of physically identifiable opposing poles 32A and 32B. A working gap G, located between poles 32A and 32B, is the location at which a growth hardware 34 for containing a crucible is positioned. Coils 36A and 36B are wrapped around poles 32A and 32B, respectively, for creating a horizontal magnetic field, i.e., generally along the X or Y-axis. External yoke 38 magnetically connects poles 32A and 32B.
What has now been discovered is that a multitude of advantages can be gained if a vertical magnetic field, i.e., generally along the Z-axis, is applied to growth hardware 34, as opposed to a horizontal field, i.e., generally along the X or Y-axis. To produce a vertical magnetic field, poles 32A and 32B must be positioned on top and bottom of chamber 30. This configuration, however, interferes with the production of dendritic web crystals. More specifically, the top pole serves as a physical barrier which prevents the extraction of the web through the top of chamber 30. Accordingly, there is a need for a magnetic generator which produces a generally vertical magnetic field without interfering with the production of web crystals.
In accordance with one aspect of the embodiments of the present invention, an apparatus for manufacturing a semiconductor substrate such as web crystals is provided. The apparatus comprises a chamber and a growth hardware assembly located in the chamber. The growth hardware assembly is used for growing the substrate. A magnetic field generator encircles the perimeter of the chamber. The magnetic field generator is used for providing a magnetic field during the growth process. The chamber includes a vertical axis (illustrated as Z-axis) which can be generally defined by the longitudinal direction of crystal growth. The magnetic field generator produces a magnetic field that is generally in this vertical direction.
In one embodiment the magnetic field generator comprises a coil assembly which encircles the perimeter of the chamber. The coil assembly includes at least one winding element for receiving an electrical current. A cooling plate is in thermal communication with the coil assembly. The cooling plate is used for transferring heat generated from electrical current passing through the winding element. The heat can be removed by running water through cooling tubes disposed in the cooling plate. The cooling tubes can be electrically isolated from the winding elements for significantly reducing or eliminating electrolysis.
A shell can at least partially enclose the magnetic field generator. The shell can be used for containing the magnetic field within the shell, for controlling the direction of the magnetic field within the chamber, and enhancing the magnetic field strength at the location of the growth hardware assembly.
In one embodiment, the shell can include a sheath body having an upper flange extending from one end the sheath body and a base flange opposing the upper flange and enclosing the other end of the sheath body. The shell can be made from a ferromagnetic material and can additionally include a field clamp member disposed within the chamber and positioned over the growth hardware assembly. The field clamp member has an opening through which a web crystal can be extracted from the growth hardware assembly. The field clamp member is in magnetic communication with the upper flange, the upper flange being positioned outside of the chamber. A transition ring can be used to magnetically couple the upper flange to the field clamp member.
In accordance with another embodiment, a field shaping plate can be disposed in the chamber for supporting the growth hardware assembly. The field shaping plate can enhance the magnetic field over the growth hardware assembly. The field shaping plate can have a variable thickness to define a selected geometrical configuration, the magnetic field strength being dependent on the geometrical configuration.
In accordance with another aspect of the embodiments of the invention, a process for manufacturing dendritic web crystals is provided. The process includes the acts of providing a chamber having a growth hardware assemblyxe2x80x94the growth hardware assembly containing a melt; growing a substrate from the melt; and applying a magnetic field to the melt during the act of growing, wherein said magnetic field is applied in the longitudinal direction of the growth within the chamber. The magnetic field generator circumscribes the perimeter of the chamber for applying the magnetic field to the melt.